Physicists
find a new, striking difference between matter and
antimatter

An
international collaboration of physicists conducting
experiments at the Department of Energy's Stanford
Linear Accelerator Center (SLAC) has discovered a
second fundamental difference between the behavior of
matter and that of antimatter. They observed this
intriguing phenomenon -- known as charge-parity (CP)
violation and first seen decades ago in experiments
with another particle -- in disintegrations of heavy,
short-lived subatomic particles called B mesons. The
collaboration reported its result in a paper
submitted July 5 for publication in Physical
Review Letters, a leading scientific journal.

"After
37 years of searching for further examples of CP
violation, physicists now know that there are at
least two kinds of subatomic particles that exhibit
this puzzling phenomenon, thought to be responsible
for the great preponderance of matter in the
Universe," said Princeton University physicist
Stewart Smith, spokesman of the collaboration.
"We are poised for further discoveries that
should open up new directions for particle
physics."

The
international collaboration includes more than 600
scientists and engineers from 73 institutions in
Canada, China, France, Germany, Great Britain, Italy,
Norway, Russia and the United States. They built and
have been operating the sophisticated 1,200-ton
detector, named BABAR, which was used to make the
discovery.

The
detector records subtle distinctions between decays
of B mesons and those of their antimatter
counterparts, called anti-B mesons. Both are more
than five times heavier than protons and survive just
over a trillionth of a second. Physicists employed
the detector to observe an unmistakable difference,
or asymmetry, between the rates at which B and anti-B
mesons decay into a special set of specific final
states.

From
these measurements, they calculated a parameter
called sin 2b (sine two beta), which expresses the
degree of asymmetry between matter and antimatter. A
non-zero value of this parameter is clear evidence
for CP violation among B mesons.

In the
paper just submitted, the BABAR collaboration
reported measuring a value of sin 2b = 0.59 ± 0.14,
which is substantially different from zero. There are
now fewer than 3 chances in 100,000 that the actual,
physical asymmetry could be consistent with zero.

This
BABAR result is easily the most precise measurement
of sin 2b reported to date. Earlier measurements made
at Fermi National Accelerator Laboratory, the
Japanese National Laboratory for High-Energy
Accelerator Research (KEK), and at SLAC by the BABAR
collaboration are consistent with the present result
but not as accurate. The value just reported agrees
with expectations based on the Standard Model,
today's dominant theory of particle physics.

The
precision of the BABAR result was made possible by
the outstanding performance of the PEP-II B Factory
at SLAC. Built in collaboration with the Energy
Department's Lawrence Berkeley and Lawrence Livermore
National Laboratories, this pair of 2.2-kilometer
storage rings collides unequal-energy beams of
electrons and their antimatter counterparts, called
positrons. Piermaria Oddone, now deputy director of
the Berkeley lab, first proposed this innovative
experimental approach, which greatly enhances the
accuracy of many B meson measurements.

"The
B Factory has performed beyond expectations,
permitting the BABAR collaboration to make the
world-class measurements on B mesons," said SLAC
Director Jonathan Dorfan, who played a pivotal role
in designing and building this particle collider.
Since it began operating in June 1999, the B Factory
has produced more than 32 million pairs of B mesons,
from which data the present BABAR result was
extracted.

The
mysterious phenomenon of CP violation was first
discovered in a 1964 experiment led by James Cronin
and Val Fitch at the Brookhaven National Laboratory.
Their group observed this behavior in decays of
subatomic particles called K mesons, which are about
one tenth as heavy as B mesons and live much longer;
the two physicists shared a Nobel Prize for the
discovery.

Several
observations of CP violation have since occurred in
experiments with K mesons. But until the recent BABAR
discovery, no other subatomic particles had clearly
exhibited this exceedingly rare phenomenon. Having
this second striking example of CP violation should
aid theorists trying to understand what causes it.

Scientists
are interested in this puzzling behavior because it
can help explain the abundance of matter in the
Universe. In 1967, Russian theorist Andrei Sakharov
used CP violation to suggest how the present
matter-dominated Universe could have emerged from one
that contained exactly equal amounts of matter and
antimatter during the earliest moments of the Big
Bang.

The
Stanford Linear Accelerator Center is a national
laboratory for high-energy physics and
synchrotron-radiation research operated by Stanford
University on behalf of the U.S. Department of
Energy. The Department's Office of Science funded the
construction of the B Factory at $177 million and
contributed about 60 percent of the cost of the BABAR
detector, with the remainder coming from foreign
sources. "The foreign contributions to this
experiment, both monetary and scientific, have been
absolutely crucial to its success," noted former
BABAR spokesman David Hitlin of the California
Institute of Technology.